Chemical in petroleum at once exploding (6)
I believe the answer is:
octane
'chemical in petroleum' is the definition.
'at once exploding' is the wordplay.
'exploding' is an anagram indicator.
'at'+'once'='atonce'
'atonce' is an anagram of 'OCTANE'.
(Other definitions for octane that I've seen before include "Anti-knock element in petrol" , "One cat (anag.)" , "Hydrocarbon found in petrol - at once (anag)" , "Anti-knock rating" , "A liquid hydrocarbon found in petroleum" .)